Mass spectrometry (MS) is an analytical technology used to identify the types and amounts of chemicals present in a sample. This determination is made by measuring the mass-to-charge ratio and abundance of the gas-phase ions generated from a sample. Mass spectrometers generally include three components: an ion source, a mass analyzer and a detector. In a typical MS analysis a portion of a sample is ionized. A wide variety of ionization techniques exist including electron bombardment, chemical ionization, and laser desorption, among others. The ionization process causes some of the sample's molecules to become molecular ions, and some may dissociate into smaller ions representing a portion of the original molecule. The ions thus generated are then separated by the mass analyzer according to their mass-to-charge ratio by subjecting the ions in a controlled manner to either an electric or magnetic field, or a combination of electric and magnetic fields. The ions are ultimately sent to a detector capable of sensing charged particles, such as an electron multiplier. The detector records the charge induced or the current produced as an ion passes by or contacts a surface. Results are displayed as a histogram of the relative abundance of detected ions as a function of their mass-to-charge ratio. Molecules in the sample are then identified by correlating the identified mass-to-charge ratios to chemical structures and through characteristic fragmentation patterns.
Various mass analyzers are used to separate ions, including time-of-flight analyzers, quadrupole analyzers, and ion traps. In each case a critical parameter is the shape and position of the electrodes used to establish the electric fields that control the ions for analysis. In the case of quadrupoles and ion traps the electrodes may include a curved surface that is used to establish a hyperbolic electric field essential to the performance of the analyzer. In the classical quadrupole ion trap, one electrode includes an aperture that allows an electron beam to enter the ion trapping region, and another electrode includes an aperture through which ions exit to the detector. It may also be desirable to include additional apertures to allow for modified operation of the ion trap. Unfortunately, the additional apertures further distort the surface of the endcap electrodes, and consequently the electric field generated by the electrode is non-ideal as well, diminishing the performance characteristics of the analyzer. Similarly, in the time-of-flight analyzer, the electrodes are generally flat plates having large apertures, and the electric field penetrating an aperture from one side of the electrode may adversely affect the electric field on the other side.
Generally the solution for minimizing any electric field distortion is to minimize the size of the aperture. However, limiting the cross section of the aperture also limits the passage of electrons, ions, gas molecules, and light through the aperture. It is possible in time-of-flight analyzers to utilize a large cross-section-area aperture on a planar electrode and add a high-transmission electroform mesh to the electrode surface over the aperture to preserve the electrode planarity. Practically, however, attaching the mesh is cumbersome and it is problematic to attach the mesh to the electrode without causing some distortion of the electrode planarity. For analyzers such as ion traps and quadrupoles, a mesh cannot practically be attached to curved-surface electrodes in a manner that both preserves the intended curvature of the electrode surface over the aperture and retains sufficient structural durability. Electrodes have been made using a woven mesh to provide high optical transmission while enduring some distortion to the electric field due to the irregular electrode surface.
It is desired to develop a method of establishing an aperture of any size in an electrode such that the intended electrode shape is highly conserved and structurally durable. Such a method could yield electrode apertures that are reproducible and allow the addition of apertures that have heretofore been impractical.